Lasers are used in a variety of applications, such as laser engraving, hair removal, cleaning, optical communications, and many industrial applications. Lasers are especially useful as transmitters for high-speed optical communication transmitters.
Coherent laser light may be generated by a laser diode fabricated from semiconductor materials. Laser diodes that emit light in a direction parallel to the surface of the wafer require breaking the wafer before testing, resulting in high production and packaging costs.
Surface-emitting laser diodes are preferred because they can be tested at wafer sort, before individual laser diodes are separated and packaged. Bad laser diodes are not packaged, resulting in lower costs. A widely available type of laser is known as a Vertical-Cavity Surface-Emitting Laser (VCSEL).
Electrical current flowing through the VCSEL laser diode generates laser light. However, the optical power is a non-linear function of the current flow through the VCSEL. A VCSEL device tends to have a fast turn-on (rise) time but a slow turn-off (fall) time. Parasitic capacitances in the VCSEL are slowly discharged through parasitic resistances in the VCSEL, causing the VCSEL to remain on for a period of time after it is turned off, resulting in a slow fall time.
Various laser driver circuits have been used to drive current through VCSELs. Common-cathode driver circuits are less desirable due to large parasitics on the PMOS driver transistor connected to the anode. Common-anode laser drivers are more widely used due to lower parasitics of NMOS transistors connected to the cathode.
FIGS. 1A-1B show a common-anode VCSEL driver. The positive or anode terminal of VCSEL 10 is connected to a common power supply. The current through the negative or cathode terminal of VCSEL 10 passes through transistor 16. Transistor 16 is switched by gate voltage VG to increase or decrease the current through VCSEL 10. In FIG. 1A, VG is high, and transistor 16 is on, allowing a large current to be pulled through VCSEL 10, which emits laser light at a high optical power.
In FIG. 1B, VG is switched low, turning transistor 16 off. A small bias current (not shown) may still flow through transistor 16, or through another component (not shown) to bias cathode voltage V_VCATHODE to a desired off voltage. Ideally, the light produced by VCSEL should immediately decrease to near-zero when VG is switched from high to low. However, the physical VCSEL device does not respond in an ideal manner. Instead, VCSEL 10 continues to outputs some light for a short period of time after current stops flowing through transistor 16.
This non-ideal behavior of VCSEL 10 can be modeled by parasitic capacitor 14 and parasitic resistor 12 that are each in parallel with the ideal laser diode shown as VCSEL 10. When VCSEL 10 is on (FIG. 1A), parasitic capacitor 14 is being charged by an excess of negative electrons on its lower plate. When transistor 16 turns off (FIG. 1B), the negative charge stored on parasitic capacitor 14 is discharged through parasitic resistor 12. Since parasitic resistor 12 and parasitic capacitor 14 are not real devices, but are part of the physical VCSEL device, the electrons stored on parasitic capacitor 14 really flow through the laser diode of VCSEL 10, causing VCSEL to emit light.
The amount of time that VCSEL 10 emits light after it is turned off can be modeled by the R-C time constant of parasitic capacitor 14 and parasitic resistor 12. Thus the slow turn-off of VCSEL 10 can be explained by electrons flowing from the bottom plate of parasitic capacitor 14, through parasitic resistor 12 to the power supply (anode).
FIGS. 2A-B show the effects modeled by the parasitic capacitor and resistor in a VCSEL. In FIG. 2A, the voltage across VCSEL 10 is low when the laser diode is turned off, but high when the laser diode is turned on, and a larger current is flowing through the laser diode. The rise time (TR) to turn on VCSEL 10 is relatively short. However, the fall time (TF) is longer. The voltage across VCSEL 10 decreases slowly because parasitic capacitor 14 has been charged while VCSEL was on, and much be slowly discharged through parasitic resistor 12. Parasitic capacitor 14 causes the voltage across VCSEL 10 to be larger that it ideally would be as VCSEL turns off. Optically, some light would continue to be observed from VCSEL 10 during TF. This is undesirable.
FIG. 2B shows an eye diagram. When viewing on a scope or other instrument, the rising and falling waveforms may be superimposed over each other. The fall time is somewhat slower than the rise time, as can be seen in the eye diagram. The eye diagram waveform is not exactly symmetrical, but is skewed by the slow fall time.
FIG. 3 shows voltage and current characteristics of the common-anode laser-driver circuit of FIGS. 1A-1B. When the gate voltage VG of transistor 16 is high, VCSEL 10 is on, allowing a high diode current I_DIODE to flow through. The voltage across VCSEL 10 is V_ON, which is the difference between anode voltage V_ANODE and cathode voltage V_CATHODE.
When gate voltage VG is driven low, transistor 16 turns off. Diode current that is flowing through VCSEL 10 causes the cathode voltage V_CATHODE to rise since the cathode node is no longer being discharged to ground by transistor 16. As the cathode voltage rises, the voltage across VCSEL 10 decreases, causing the diode current to decrease. Eventually the cathode voltage rises enough that the voltage across VCSEL is below its turn-on voltage VOFF and diode current stops. The diode current I_DIODE turns off slowly due to the R-C time constant of the parasitic resistor and capacitor in the physical VCSEL device.
FIG. 4 shows frequency chirping of a VCSEL device. As the frequency of operation of the VCSEL device is increased, eventually an undesirable phenomenon known as chirping occurs. As FIG. 4 shows, the optical power initially spikes too high as the VCSEL is turned on at high frequency. At very high frequencies, inherent physical characteristics of VCSEL devices cause frequency chirping.
What is desired is a laser driver circuit tailored for driving VCSELs. A VCSEL driver circuit is desired that compensates for non-linear behavior of a physical VCSEL device. A VCSEL driver circuit that compensates for the slow turn-off of a VCSEL is desirable. A VCSEL driver that can operate at high frequencies is desirable.